Fluidization in pyroclastic flows
T.H. Druitt (Université Blaise Pascal & CNRS, Clermont-Ferrand, France)
G. Bruni, P. Lettieri, J. Yates (University College, London)
Small volume pyroclastic flows are highly concentrated granular avalanches of
hot (200-700 °C) volcanic debris fluidized, or partly fluidized, by escaping
gases. They form by gravitational collapse of lava domes, by fallback of
vertical eruption columns, or by rapid sedimentation from turbulent
pyroclastic suspensions. Flows of different origins exhibit different degrees
of friction reduction by fluidization. When fluidized at high temperature,
and while being agitated mechanically (probably analagous to shear in the
natural system) some pyroclastic flow materials pass through three distinct
states as the vertical gas velocity is increased: (1) aerated, (2)
homogeneously fluidized, and (3) bubbling states. Defluidization from high
initial gas velocity, as might occur progressively in a propagating
pyroclastic flow, takes place first by bubble evacuation, then by homogeneous
hindered settling with progressive aggradation of a basal aerated layer.
Friction in the aerated layer then increases from the base up by pressure
diffusion in the loosely packed state until degassing is complete. Recent
work has placed certain constraints on the origin of the gases that generate
fluidization in pyroclastic flows. The development of physically realistic
models of pyroclastic flows offers an important challenge for the future.